Redefining the Microwave Plasma-Mediated CO₂ Reduction Efficiency Limit: The Role of O–CO₂ Association

Reduction of CO2 to CO for sustainable fuel production is studied in high-temperature (>3500 K) microwave plasma. Raman scattering and chemical kinetics modeling reveal chemistry rates with spatial resolution that explain previously reported peak energy efficiency values of 50%. The necessary product quenching is established by fast transport in the core, at frequencies of 105 s–1, facilitating rapid mass and energy transfer between products and feedstock CO2. Moreover, the resulting chemical nonequilibrium yields additional CO2 dissociation in O–CO2 association, a reaction responsible for up to 45% of CO production. Three different thermal chemistry sets are invoked to qualitatively confirm this picture. It is shown how these lack predictive accuracy in the high gas temperature regime studied, which indicates that new CO2 chemistry rate coefficients are highly desirable. Improving reactor design with the identified enhancement mechanisms in mind can increase efficiency up to the newly defined thermal limit of 70%.